• Hide the python interpreter on sessions with only one. (#148)
• Warn when programs not in the virtualenv are used, allow erroring and silencing the warning. (#147)
• Add --warn-on-external-run flag and the "external" keyword arg to session.run. (#147)
• Add nox.options which allows specifying command-line configuration in the Noxfile (#145)
• Add python_requires (>= 3.5) to setup.py.